Page 69 - Materials Chemistry, Second Edition

50 Life Cycle Assessment of Wastewater Treatment

4.1 INTRODUCTION
As sustainable energy gains momentum in today’s society, municipal wastewa-
ter shows promise as the next source of clean energy. In 2012, the Environmental
Protection Agency (EPA) found that over 14,000 publicly owned wastewater treat-
ment plants were serving 238.2 million Americans (United States Environmental
Protection Agency, 2016). In the United States, 76% of the population send their
wastewater to public facilities, while the remaining population use private septic
systems (United States Environmental Protection Agency, 2016).
When municipal wastewater is collected and sent to a centralized municipal
wastewater treatment facility, it goes through a series of treatment processes to be
cleaned and purified. The treated water is then discharged to a local body of water.
However, in addition to the cleaned water, treatment plants also produce a series of
waste streams, which are likely to facilitate ecological harm when discharged into
the environment. A commonly known waste stream produced by municipal waste-
water treatment plants is biosolids, which are mainly derived from the excess acti-
vated sludge in aeration tanks and are usually deposited to landfills, which has raised
significant concerns in waste management. Other wastes include centrate, which is
the wastewater collected from the sludge dewatering; this is not typically discharged
out of the plant directly, but recycled back into the activated sludge. As it contains
high chemical oxygen demand (COD) and nutrient levels, centrate can raise the treat-
ment loading of aeration tanks.
To reduce the waste generated, a variety of technologies have been devel-
oped and applied in wastewater treatment facilities. One of these technologies is
sludge digestion, a process that has been used for many years to stabilize raw
sludge and recover energy in sludge. In addition, the digested sludge has been
proposed to improve soil nutrients under federal and state standards. Besides con-
ventional sludge digestion, which produces bioelectricity, other technologies have
been developed in the recent past to reduce and reuse wastes generated in treat-
ment facilities. In particular, due to the rising oil prices since 2005, interest has
increased in the creation of technologies that can produce transportation biofu-
els using waste streams. Using nutrient-rich centrate, for example, algae can be
cultivated and then converted into transportation fuels for vehicles. Pyrolysis or
hydrothermal liquefaction has also been tested for converting sludge to biofuels.
A specific technology has been developed to convert the lipids in scum to biodiesel
using acid hydrolysis and glycerolysis. Rather than being perceived as waste, bio-
solids have come to be viewed as a valuable resource for water, energy, and plant
nutrient conservation (Asano et al., 2007).
A detailed description of each of the waste streams has been introduced, along
with several technologies that convert the various waste streams within the munici-
pal wastewater treatment plant to biofuels or bioenergy. More importantly, this
chapter discusses the environmental impacts and benefits of these technologies in
comparison to conventional petroleum and electricity. A life cycle assessment (LCA)
is included in this chapter as a case study, which took place at a wastewater treatment
plant located in Saint Paul, MN USA that integrated various technologies of biofuel/
bioenergy production to recycle and reuse waste streams.